1. Field of the Invention
This invention relates to a dust pre-removal method in a dry moving bed type adsorption tower and more particularly, it is concerned with a method of reducing the dust concentration in a gas passing through a granular material bed in an apparatus for contacting a gas with a granular material, e.g. a dry moving bed type adsorption tower used for removal of SO.sub.x and NO.sub.x from waste gases.
2. Description of the Prior Art
One example of a method for the removal of SO.sub.x and NO.sub.x using an ordinary SO.sub.x and NO.sub.x removal apparatus of dry moving bed type will now be illustrated by a flow sheet as shown in FIG. 1.
Waste gas 1 from boilers, etc. is introduced into adsorption tower 2, brought into contact with a granular carbonaceous adsorbent 3 packed and held in the tower, thus subjected to reactions for removal of SO.sub.x and NO.sub.x and to removal of dust and exhausted through a stack (not shown) in the air. On the other hand, carbonaceous adsorbent 3 is continuously withdrawn from the lower part of adsorption tower 2 by feeder 5 while holding a constant level of powder in hopper 4 over the upper part of adsorption tower 2 and thus forming a moving bed in adsorption tower 2, and SO.sub.2 in the gas is adsorbed in the form of H.sub.2 SO.sub.4 during the course of entering and leaving adsorption tower 2, while dusts and the like are also adsorbed. The adsorbent withdrawn from feeder 5 is then fed to hopper 7 for regeneration tower 8 by means of bucket conveyor 6, introduced into regeneration tower 8 while holding a constant level of powder in hopper 7 and continuously withdrawn therefrom by feeder 9. In regeneration tower 8, the adsorbent is heated to decompose H.sub.2 SO.sub.4 adsorbed into SO.sub.2 and thus released SO.sub.2 gas 13 is fed to another plant for byproducts (not shown). The adsorbent withdrawn by feeder 9 is subjected to vibrating screen 10 where dusts in the waste gas and abrasive powders formed during movement, adhered to the adsorbent, are separated, and then returned to hopper 4 by means of bucket conveyor 11. Since the quantity of the adsorbent decreases due to the chemical reactions in regeneration tower 8, the withdrawal from vibrating screen 10 and the entrainment with the gas, the decreased adsorbent should be made up from make-up hopper 12.
A waste gas from a boiler or the like is ordinarily supplied to an adsorbing tower via a dust collector, but generally has a dust concentration of about 200-400 mg/Nm.sup.3. The dust removal or dedusting capacity of an adsorption tower is generally so large that in a test of flowing a gas at a linear velocity of 0.15 Nm/sec through a moving bed with a thickness of 1.5 m using an adsorbent with a mean grain size of about 10 mm, for example, at the outlet of the adsorption tower, there is little dust having the same composition as that at the inlet thereof. However, the dust concentration itself at the outlet of an adsorption tower is not so small, the most part of which consists of a fine powder of carbonaceous adsorbent. Since the dust concentration of a waste gas exhausted in the air has severely been regulated independently of the dust composition, it is required, in order to overcome this regulation, to provide a dust collector even at the rear of an adsorption tower in many cases.
A fine powder of an adsorbent contained in an outlet gas of an adsorption tower is produced by abrasion and cracking due to collision of the grains each other at various positions in the cycle line of the adsorbent. Thus, sieving is carried out by installing a vibrating screen, but it is difficult to sufficiently remove a fine powder of about several microns by an ordinary vibrating screen and a large part of the fine powder passes with larger grains. That is, the sum of a powder passing through a vibrating screen and a powder produced from the vibrating screen to the adsorption tower inlet enter the adsorption tower, where a fine powder produced by the movement of the adsorbent is further added, but the quantity of this fine powder is very little because the moving speed of the adsorbent is very low in the adsorption tower. Such a fine powder can well be removed by rinsing out with a gas and accordingly, the powder rinsed and removed in an adsorption tower results in increase of the dust concentration at the outlet of the adsorption tower.
When the dust concentration in a waste gas is not less than the regulation value at the outlet of an adsorption tower, a dust collector must be installed, the size of which is dependent upon the quantity of a waste gas to be processed and in general, substantially similar to that of the adsorption tower. Such a dust collector not only occupies a large space, but also presents large costs for installation and maintenance.